The invention has particular application to heat recovery in building ventilation systems but is also applicable to industrial processes and the like.
The heating and cooling of buildings, particularly at high latitudes, is a major cause of high consumption of fossil fuels. With the escalating global concerns about pollution and climatic changes due to the accumulation of green house gases in our atmosphere, and the rising concerns over reliable, reasonable cost oil supplies, more attention is being given to heat recovery from exhaust air. In a typical run-around building heat recovery system, heat is recovered from exhaust air using an exhaust heat exchanger and is added to incoming fresh air using a supply heat exchanger. The medium for heat transport is usually an aqueous glycol solution of 30% glycerine by weight in order to prevent freezing at temperatures below 0.degree. C. These systems are also used to cool incoming fresh air in air-conditioned buildings.
The new ASHRAE standard for indoor air quality (ASHRAE standard 62-89) calls for increasing air ventilation rates in most buildings by a factor of three when compared with the former standard. This results in a much higher cost of energy to heat (or cool) the incoming air into buildings. While the efficiency of heat recovery ventilators in small buildings and residential dwellings may now reach as high as 90%, it rarely exceeds 50% in large commercial buildings due to many factors. These include an excessively high glycol content in the coupling fluid, producing a high thermal capacity rate, and hence reducing the thermal transport rate to the air side of each heat exchanger. Another factor is condensation and frost accumulation on the exhaust heat exchanger. One way of significantly increasing the overall effectiveness of the system would be to increase the liquid-side heat transfer coefficient.
Additionally, the optimum overall efficiency of a run-around system could be improved if the ratio between the heat capacity rate of the liquid side to the heat capacity rate of air could be brought closer to unity. In a typical heat exchanger, where a single-phase liquid flow is commonly used, this ratio is usually two or even higher.
A parameter known as the "Number of Transfer Units" is commonly used in the equations used for calculating the effectiveness of a heat exchanger. This parameter, N, is calculated by multiplying the overall heat transfer coefficient, U, in a heat exchanger (explained in the following detailed description) by the heat exchanger total heat transfer area and dividing by the minimum heat capacity of either the liquid or the air stream. Since the minimum heat capacity in the system is usually that of the supply air, no dramatic changes could be made to this value. However, the number of transfer units in each heat exchanger, and hence the overall effectiveness of the run-around system, could be significantly increased if the overall heat transfer coefficient U is increased.
It has now been found that a two-phase coupling fluid may be used to achieve the desired objectives.